边际效用递减究竟是不是边际替代率递减的原因？

二者毫无关系
边效递减是基数理论的假设，MRS是序数理论基础。序数理论的反对效用的主观测量，既然不能测量，也就不存在所谓递减问题。
有同学提出MRS=MUX/MUY来解释，但记住，经济学研究中，数学上正确的理论上未必正确。在前面的公式中，如果X增加，Y减少，既然Y在发生变化，如何保证MUX递减？边校递减的前提是其他商品不变。
所以，边际替代率递减，即无差异曲线的严格凸性是无需证明的，是序数理论的假设前提。

to 9楼
边际效用递减指的是单个商品的效用随着重复刺激的递减。
而货币不是商品，而是对一组商品的潜在权力。这个问题比较复杂，但是只要相信理性人都是先满足效用最大的需求的话，那么货币也是边际效用递减的。

不是，

不是，可以构造出边际效用递增的效用函数，具有边际替代率递减的性质

Diminishing marginal utility MUx is the additional utility from consuming one more unit of good X (all else equal).
Diminishing Marginal Rate of Substitution is as a consumer has more of good X and less of good y, the consumer is willing to give up less y for an additional unit of x.
MRSx,y= - deltaX/deltaY holding U equal, which is the same as the negative value of the slope of the indifference curve  = the rate at which consumer is willing to trade Y for X.

U(X,Y)   dU=dU/dx*dx+dU/dy*dy  because you are moving on an indifference curve, dU=0
0=MUx dx+MUy dy
which equals to -dy/dx=MUx/MUy
MRSx,y=MUx/MUy

Hope this helps. It is not a relationship of who is the base of whom, but it is a relationship derived from mathematical equations and the indifference curve.

I might be wrong!

My definition above shows pretty clear that the definition of the two-MU and MRS are different! But, on the graph of indifference curve, these two definitions do have a mathematical relationship, they do not have a relationship of who determines whom just because they are different ideas!

是的

偏好分析中没有边际效用这一说吧。做个CD函数表示偏好类型，做单调变换，“边际效用”没有意义。

考虑当中